This invention relates to the field of cementing well conduits in a well conductor during well completion operations.
The cementing of well conduits in a well conductor is old and well known. For an example of a description and illustration of the prior art and for details of well known cementing apparatus and procedures, specific reference for incorporation herein is made to Pages 296-320 of Volume I and Pages 1890-1944 of Volume II of the "Composite Catalog of Oil Field Equipment and Services", 30th Revision, 1972-73, published by World Oil, Houston, Tex.
Known prior art methods of cementing a well conduit in a subsea well conductor included making up a tubular string from a plurality of sections of well conduit having the same size for extending the conduit from a work surface to inside a well conductor which was positioned in the earth. Since several concentrically mounted conduits may be cemented in a well conductor, this practice would include use of many sections of each size conduit as the depth of the well increased and in the case of offshore wells, as the depth of water increased. A plug container was positioned at the drilling platform at the upper end of the tubular string to provide one or more plugs for sometimes clearing drilling mud from the string before cementing and for clearing the cement from the string at the end of the cementing operation. Such a procedure would require the string to have substantially the same diameter throughout its length to permit passage of plugs having sufficient size through the entire length of the string to be cemented.
A usual practice in drilling offshore wells has been to first drive or jet into the sea floor a large conductor casing, for example of 30 inch diameter, having a casing head until the casing head rests near the floor which floor may be at a considerable depth from the work surface. A next step included concentrically mounting and cementing a plurality of conduits within the conducting casing which conduits may be, for example, 20 inches, 133/8 inches and 9 and 5/8 inches in diameter. One example of this type of completion operation is disclosed in U.S. Pat. No. 3,847,215, issued Nov. 12, 1974 to D. P. Herd which patent is incorporated herein by specific reference. Centralizers and scratchers are often used to center the conduits and clear the well conductor of obstructions such as gelled mud, debris and filter cake. U.S. Pat. No. 3,828,852 issued Aug. 13, 1974 to Charles G. Delano discloses that it is known that a conduit may be rotated and reciprocated to prevent sticking when lowering and positioning in the well conductor.
As noted above, since a plug clears the conduit mounted in the conductor casing from cement or other fluid the plug should be substantially the same diameter as the conduit. Accordingly, the entire tubular string, which constitutes risers would also have to be a sufficient diameter to permit the cementing plug to travel from the plug retainer mounted at the drilling platform through the tubular string to the lower portion of the string in the well conductor. This type of procedure may require a large quantity of each size of conduit which is cemented in the conductor casing, as well as the length of casing forming the riser. In the case of offshore wells, this also may require supplying an excess amount of casing for use as a riser for each size of casing to be cemented. After cementing it was necessary to disassemble the riser pipe and then transport the riser casing back to store. Such efforts were expensive and delayed the resumption of drilling operation.
It is known in the art that it may take as much as a 24 hour stand-by period for the cement to set up to properly secure a conduit in a well. During this stand-by period it may be necessary for the casing and cementing crew to remain at the drilling rig to disassemble the riser, particularly in the case of an offshore rig, as well as retaining the casing elevators and casing power tongs which are used on the riser and which are usually rented on a daily basis. The use of the riser cementing method may require an additional cost of one or more days rental for the casing and cementing equipment and of the crew since the crew must wait until the cement sets up to disconnect the riser section before retrieving their equipment and leaving the work platform.